The present invention relates to a compound catalyst system useful for synthesizing polyethylenes or copolymers of ethylene having bimodal or broad molecular weight distributions, a process for preparing the catalyst component used in the compound catalyst system according to the present invention and use of the compound catalyst system according to the present invention.
It is well known that the polymers obtained by using metallocene catalyst systems have various advantages, but their molecular weight distributions are very narrow, for example, MWD being only 2-3, and thus it is very difficult to process such polymers. Many attempts have been made with respect to the compound catalyst systems in order to improve the processability of the polymers obtained by using such catalyst systems, i.e. broaden their molecular weight distributions. By xe2x80x9ccompound catalyst systemxe2x80x9d, it means that two or more different active centers are compounded together, thereby synthesizing polymers having excellent balance of processability and mechanical properties.
The types of the compound catalyst systems primarily include metallocene/metallocene compound catalyst systems and metallocene/Ziegler-Natta compound catalyst systems, with the latter being particularly preferred. By compounding a metallocene catalyst with a Ziegler-Natta catalyst, it is possible to overcome the disadvantages of the metallocene catalyst system, such as a single active center, narrow molecular weight distribution of the resulting polymers and poor processability, while maintaining its advantages, such as good stereoregularity of the resulting polymers, good copolymerizability and the like. For example, European Patent Application Publication 0 447 070 A1 discloses a solid catalyst suitable for polymerizing olefins in a heterogeneous process, comprising (1) a solid support containing from 80 to 99.5 mol % of magnesium dichloride and from 0.5 to 20 mol % of an electron-donor compound free from labile hydrogen, (2) an electron-donor compound containing labile hydrogen, (3) a zirconium metallocene, (4) at least one halide of a transition metal chosen from titanium and vanadium, and (5) an organoaluminum compound, preferably an aluminoxane compound. The catalyst disclosed therein is prepared by supporting zirconium and titanium or vanadium onto the support formed from magnesium dichloride and is suitable for the heterogeneous polymerization of olefins, and however, such a catalyst has a relatively low activity and the molecular weight distribution (Mw/Mn) of the resulting polymer is only about 4, which is still relatively narrow. European Patent Application Publication 0 447 071 A1 discloses a compound solid catalyst suitable for polymerizing olefins, comprising (1) a support containing from 90 to 99.9mol % of magnesium dichloride and from 0.1 to 10 mol % of an electron-donor compound free from labile hydrogen, (2) a zirconium metallocene, (3) at least one halide of a transition metal chosen from titanium and vanadium, and (4) an organoaluminum compound, preferably an aluminoxane compound. The catalyst disclosed therein is prepared by supporting zirconium and titanium or vanadium onto the support formed from magnesium dichloride and is suitable for the heterogeneous polymerization of olefins, and can produce polymers having better particle morphology (Dm/Dn: 1.5; bulk density: 0.3-0.4 g/cm3). Although such a catalyst has a higher ratio of Zr to Ti, for example from 2 to 20, it has a relatively low activity and the molecular weight distribution (Mw/Mn) of the resulting polymer is only about 5, which is still relatively narrow. The reason may be that part of zirconium supported is inactive for being covered, thus decreasing the activity of the whole catalyst.
Chinese Patent Application Publication CN1138589A discloses a solid compound catalyst and a catalyst system useful for polymerizing olefins. The solid compound catalyst comprises a metallocene compound, a halide of a transition metal selected from elements of Group IIIB, IVB, VB and VIB of the Periodic Table, and magnesium halide. Although the activity of the whole catalyst has been largely improved and the resulting polymer has a molecular weight distribution, Mw/Mn, of 3.5 to 9.8, no bimodal molecular weight distribution is found. It shows that the metallocene component in the catalyst fails to fully exert its activity.
An object of the present invention is to provide a compound catalyst system useful for synthesizing polyethylenes or copolymers of ethylene having bimodal or broad molecular weight distributions, which overcomes the disadvantages associated with the prior art compound catalyst systems and is prepared by incorporating an organic alcohol as an electron-donor compound and an aluminoxane compound into the prior art compound catalyst systems, thereby promoting the activities of the two active centers, which leads to not only a large improvement in the activity of the catalyst system, but also the formation of polyethylenes and copolymers of ethylene having bimodal or broad molecular weight distributions, which in turn gives polymers having excellent processability and mechanical properties.
Another object of the present invention is to provide a process for preparing the solid catalyst component used in the compound catalyst system according to the present invention.
Still another object of the present invention is to synthesize polyethylenes or copolymers of ethylene having bimodal or broad molecular weight distributions by using the compound catalyst system according to the present invention.
These and other objects, features and advantages of the present invention will be readily apparent to those skilled in the art from the following description taken in connection with the accompanying drawings.